Ok, here is an idea:
1. i can help you install the Courier Mail Server
http://www.courier-mta.org/ on one of your dedicated servers
2. with it, you can virtually manage any amount of users, there is a project
http://sourceforge.net/projects/courier-mailadm/ that does just this through a php/mysql interface
3. we install a webclient of your choice, even Roundcube for Joomla will work since you will be administrating users through the Courier server with a script we could provide for automatic account creation
4. after the mail server is up and running, you will easily be able to add any modifications you want to it by extending the web scripts based on it
The benefits of this solution are multiple, as you may also check on their project page for the provided features!
Ovidiu
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